1. Field of the Invention
The present invention relates to a wireless communication system and user equipment providing wireless communication services, and a method of transmitting and receiving data between a terminal and a base station in an evolved Universal Mobile Telecommunications System (UMTS) that has evolved from a Universal Mobile Telecommunications System (UMTS), a Long Term Evolution (LTE) system or a LTE-A (LTE-Advanced) system, and more particularly, to a method of receiving point-to-multipoint service data without a data loss.
2. Description of the Related Art
FIG. 1 shows a network structure of the E-UMTS, a mobile communication system, applicable to the related art and the present invention. The E-UMTS system has been evolved from the UMTS system, for which the 3GPP is proceeding with the preparation of the basic specifications. The E-UMTS system may be classified as the LTE (Long Term Evolution) system.
The E-UMTS network may be divided into an evolved-UMTS terrestrial radio access network (E-UTRAN) and a core network (CN). The E-UTRAN includes a terminal (referred to as ‘UE (User Equipment), hereinafter), a base station (referred to as an eNode B, hereinafter), a serving gateway (S-GW) located at a termination of a network and connected to an external network, and a mobility management entity (MME) superintending mobility of the UE. One or more cells may exist for a single eNode B.
FIGS. 2 and 3 illustrate a radio interface protocol architecture based on a 3GPP radio access network specification between the UE and the base station. The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane for transmitting data information and a control plane for transmitting control signals (signaling). The protocol layers can be divided into the first layer (L1), the second layer (L2), and the third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model widely known in communication systems.
The radio protocol control plane in FIG. 2 and each layer of the radio protocol user plane in FIG. 3 will now be described.
The physical layer, namely, the first layer (L1), provides an information transfer service to an upper layer by using a physical channel. The physical layer is connected to an upper layer called a medium access control (MAC) layer via a transport channel, and data is transferred between the MAC layer and the physical layer via the transport channel. Meanwhile, between different physical layers, namely, between a physical layer of a transmitting side and that of a receiving side, data is transferred via a physical channel.
The MAC layer of the second layer provides a service to a radio link control (RLC) layer, its upper layer, via a logical channel. An RLC layer of the second layer may support reliable data transmissions. A PDCP layer of the second layer performs a header compression function to reduce the size of a header of an IP packet including sizable unnecessary control information, to thereby effectively transmit an IP packet such as IPv4 or IPv6 in a radio interface with a relatively small bandwidth.
A radio resource control (RRC) layer located at the lowest portion of the third layer is defined only in the control plane and handles the controlling of logical channels, transport channels and physical channels in relation to configuration, reconfiguration and release of radio bearers (RBs). The radio bearer refers to a service provided by the second layer (L2) for data transmission between the UE and the UTRAN.
Hereinafter, the RLC layer will be explained in more detail. As mentioned above, the RLC layer operates in three modes, TM, UM, and AM. Since the RLC layer performs a simple function in the TM, only the UM and AM will be explained.
The UM RLC generates each Packet Data Unit (PDU) with a PDU header including a Sequence Number (SN), thereby allowing a receiving side to know which PDU has been lost while being transmitted. Accordingly, the UM RLC transmits broadcast/multicast data or transmits real-time packet data such as voice (e.g., VoIP) of a Packet Service domain (PS domain) or streaming on a user plane. Also, on a control plane, the UM RLC transmits, to a specific terminal or specific terminal group in a cell, an RRC message requiring no response for reception acknowledgement.
Similar to the UM RLC, the AM RLC generates each PDU with a PDU header including a Sequence Number (SN). Differently from the UM RLC, in the AM RLC, a receiving side performs acknowledgement for PDUs transmitted from a sending side. In the AM RLC, the reason why the receiving side performs acknowledgement is to request the sending side to retransmit a PDU if the receiving side fails to receive the PDU. The re-transmission function is the main characteristic part of the AM RLC. The AM RLC aims to guarantee error-free data transmission using the re-transmission function. To this end, the AM RLC handles transmission of non-real time packet data such as TCP/IP of PS domain on the user plane, and transmits an RRC message that necessarily requires a reception acknowledgement among RRC message transmitted to a specific terminal in a cell on the control plane.
In terms of directionality, the UM RLC is used for uni-directional communications, while the AM RLC is used for bi-directional communications due to feedback from the receiving side. The UM RLC is different from the AM RLC in the aspect of configuration. The UM RLC and the AM RLC are different in terms of structural aspect: the UM RLC is that a single RLC entity has only one structure of transmission or reception but the AM RLC is that both a sending side and a receiving side exist in a single RLC entity.
The AM RLC is complicated due to its re-transmission function for data. The AM RLC is provided with a retransmission buffer as well as a transmission/reception buffer for retransmission management. The AM RLC performs many functions, e.g., usage of a transmission/reception window for flow control, polling to request a status information (status report) from a receiving side of a peer RLC entity by a sending side, a receiving side's status report informing about its buffer status to a sending side of a peer RLC entity, and generating of a status PDU to transmit status information, or the like. In order to support those functions, the AM RLC requires to have various protocol parameters, status variables, and timers. The PDUs used for controlling data transmission in the AM RLC, such as the status report, a status PDU, or the like, are called Control PDUs, and the PDUs used for transferring user data are called Data PDUs.
In the AM RLC, the RLC Data PDU is further divided into an AMD PDU and an AMD PDU segment. The AMD PDU segment has a portion of data belonging to the AMD PDU. In the LTE system, a maximum size of a data block transmitted by the terminal may vary at each transmission. For instance, having generated and transmitted an AMD PDU having a size of 200 bytes at a certain time period, a sending side AM RLC entity is required to retransmit the AMD PDU since it has received a NACK from a receiving side AM RLC. Here, if a maximum size of a data block which can be actually transmitted is assumed 100 bytes, the AMD PDU cannot be retransmitted in its original form. To solve this problem, the AMD PDU segments are used. The AMD PDU segments refer to the AMD PDU divided into smaller units. During such process, the sending side AM RLC entity divides the AMD DPU into the AMD PDU segments so as to transmit the same over a certain period of time. Then, the receiving side AM RLC entity decodes the AMD PDU from the received AMD PDU segments.
FIG. 4 is an exemplary view illustrating a procedure for a HARQ operation in Acknowledged Mode Radio Link Control (AM RLC).
As shown in the FIG. 4, a HARQ operation is performed in a MAC layer for effective data transmission, and a detail description of the HARQ operation will be given as following.
Firstly, an entity in transmitting side transmits a AMD PDU 1 and a AMD PDU 2 to an entity in receiving side. Alternatively, the transmitting entity may generate a AMD PDU having 200 bytes, and then the transmitting entity may divide the AMD PDU into two AMD PDUs (i.e., AMD PDU 1, AMD PDU 2) each having 100 bytes. Thereafter, the transmitting entity may transmit the AMD PDU 2 to the receiving entity after transmitting the AMD PDU 1 completely.
In the HARQ operation, if the receiving entity receives the AMD PDU 1 successfully from the transmitting entity, the receiving entity may transmit a ACK signal to the transmitting entity. Or, if the receiving entity fails to receive the AMD PDU 1 from the transmitting entity, the receiving entity may transmit a NACK signal to the transmitting entity. In case that the transmitting side receives the NACK signal from the receiving entity, the transmitting side may retransmit the AMD PDU 1 to the receiving side.